You don’t have to use OOP in Tkinter scripts, but it can definitely help. As we just saw, Tkinter GUIs are built up as class-instance object trees. Here’s another way Python’s OOP features can be applied to GUI models: specializing widgets by inheritance. Example 8-18 builds the window in Figure 8-21.
Example 8-18. PP3EGuiIntrogui5.py
from Tkinter import * class HelloButton(Button): def _ _init_ _(self, parent=None, **config): # add callback method Button._ _init_ _(self, parent, config) # and pack myself self.pack( ) self.config(command=self.callback) def callback(self): # default press action print 'Goodbye world...' # replace in subclasses self.quit( ) if _ _name_ _ == '_ _main_ _': HelloButton(text='Hello subclass world').mainloop( )
This example isn’t anything special to look at: it just displays
a single button that, when pressed, prints a message and exits. But
this time, it is a button widget we created on our own. The HelloButton
class inherits everything from
the Tkinter Button
class, but adds
a callback
method and constructor
logic to set the command
option to
self.callback
, a bound method of
the instance. When the button is pressed this time, the new widget
class’s callback
method, not a
simple function, is invoked.
The **config
argument here is
assigned unmatched keyword arguments; they’re passed along to the
Button
constructor. We met the
config
widget method called in
HelloButton
’s constructor earlier;
it is just an alternative way to pass configuration options after the
fact (instead of passing constructor arguments).
So what’s the point of subclassing widgets like this? It allows
widgets to be configured by subclassing instead of by passing in
options. HelloButton
is a true
button; we pass in configuration options as usual when one is made.
But we can also specify callback handlers by overriding the callback
method in subclasses, as shown in
Example 8-19.
Example 8-19. PP3EGuiIntrogui5b.py
from gui5 import HelloButton class MyButton(HelloButton): # subclass HelloButton def callback(self): # redefine press-handler method print "Ignoring press!..." if _ _name_ _ == '_ _main_ _': MyButton(None, text='Hello subclass world').mainloop( )
Instead of exiting, this MyButton
button, when pressed, prints to
stdout
and stays up. Here is its
standard output after being pressed a few times:
C:PP2ndEdexamplesPP3EGuiIntro>python gui5b.py
Ignoring press!...
Ignoring press!...
Ignoring press!...
Ignoring press!...
Whether it’s simpler to customize widgets by subclassing or passing in options is probably a matter of taste. But the point to notice is that Tk becomes truly object oriented in Python, just because Python is object oriented: we can specialize widget classes using normal class-based object-oriented techniques. The next example provides yet another way to arrange for specialization.
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